WASHINGTON - The Pentagon will increase the number of U.S. troops in Iraq to about 145,000 this summer, from the current 140,000, in recognition of the continued difficulty coalition forces are having in providing security leading up to the hand-over of political power to Iraqis on June 30.

The 5,000 new troops will come from the Marine Corps. A deployment originally planned for this fall will be moved up to August. The first troops in that contingent -- 2,200 Marines from the 11th Marine Expeditionary Unit -- have already left their home base in San Diego for Iraq. The remainder will come from the 24th Marine Expeditionary Unit, based in North Carolina.

Overall, U.S. force levels in Iraq could rise even higher than 145,000. Marines already in Iraq could have their tours extended to overlap with incoming replacements, said Lt. Gen. Robert Magnus, deputy commandant for programs and resources. How long the Marines now due to rotate home will be forced to stay "depends on how well the Iraqi forces come along and whether there are more troops, or less troops, from foreign countries," Magnus told reporters Tuesday.

Pentagon spokesman Bryan Whitman said the increase in the troop level in Iraq is routine. "As you bring troops in to replace troops that are there, you will have a spike in the total numbers as you do that transition," he said. But Whitman could not say when the number will drop back to the current level or lower.

The Marines headed to Iraq had been scheduled to replace Army soldiers there, but those soldiers will stay for now.

The accelerated deployment of the Marines and the rise in the overall number of troops in Iraq come at a time when the White House had hoped to be reducing the number of troops in the summer of an election year, not increasing it.

Pentagon planners had originally hoped to shrink the number of U.S. troops in Iraq to 105,000 by this summer, but commanders asked for more troops to battle the tenacious insurgency there.

No Army units are scheduled to leave the region before August. The first Army units that are scheduled to leave are elements of the 1st Armored Division and the 2nd Armored Calvary Regiment; they were originally scheduled to leave in late April but had their one-year tours extended for 90 days.

At least one of those Army units might face a second extension. On Monday, Gen. Richard Myers, chairman of the Joint Chiefs of Staff, said he could not rule out keeping soldiers from the 1st Armored Division in Iraq beyond the current three-month extension.

"I think (another extension in Iraq) is unlikely, but in the end, we are going to have to do what is needed to be done. Never say never," Myers said Monday during a visit to Wiesbaden, Germany, where the 1st Armored Division is based. "The decision to extend the 1st Armored Division was very difficult, but they are also involved in a very important mission."

Magnus said the request for more Marines, who are traditionally used as strike forces and not occupation forces, will add to pressure that has already pushed the Marines to the point where they will begin having trouble sustaining overseas combat operations at the current level.

"We are essentially tapped out for Marine aviation," Magnus said. "If you don't have Marines and don't have Marines that are ready, you might as well not be in the business."

Because of the accelerated deployment, Magnus said he is "not sure what we will do in 2005" to meet any demands for more troops. "I have to fight the war in 2004," he said, painting a bleak picture of a Corps whose equipment is breaking down and that cannot respond quickly to a crisis in some other part of the world.

The Marines have more than 176,000 active-duty troops. In addition to the 25,000 currently in the Iraq mission, there are also 4,000 Marines in Afghanistan, 1,500 in Haiti and 1,600 in Africa.

"The Marines have not been in a prolonged combat like this since Vietnam," Magnus said. "They can't do that indefinitely."